US5796313A - Low power programmable ring oscillator - Google Patents
Low power programmable ring oscillator Download PDFInfo
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- US5796313A US5796313A US08/639,281 US63928196A US5796313A US 5796313 A US5796313 A US 5796313A US 63928196 A US63928196 A US 63928196A US 5796313 A US5796313 A US 5796313A
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03K—PULSE TECHNIQUE
- H03K3/00—Circuits for generating electric pulses; Monostable, bistable or multistable circuits
- H03K3/02—Generators characterised by the type of circuit or by the means used for producing pulses
- H03K3/027—Generators characterised by the type of circuit or by the means used for producing pulses by the use of logic circuits, with internal or external positive feedback
- H03K3/03—Astable circuits
- H03K3/0315—Ring oscillators
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03L—AUTOMATIC CONTROL, STARTING, SYNCHRONISATION OR STABILISATION OF GENERATORS OF ELECTRONIC OSCILLATIONS OR PULSES
- H03L1/00—Stabilisation of generator output against variations of physical values, e.g. power supply
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- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03L—AUTOMATIC CONTROL, STARTING, SYNCHRONISATION OR STABILISATION OF GENERATORS OF ELECTRONIC OSCILLATIONS OR PULSES
- H03L3/00—Starting of generators
Definitions
- the present invention relates to integrated circuit ring oscillators and in particular to low power programmable ring oscillators whose frequency of oscillation is independent of variations in supply voltage, temperature and process technology.
- Ring oscillator circuits are well known in the art.
- a prior art example of a ring oscillator is illustrated in FIG. 1.
- One way of constructing a ring oscillator is to cascade an odd number of inverters 30 together. The circuit will oscillate at a frequency which is typically dependent on the capacitance of the circuit, the delays associated with the circuit components, variations in supply voltage, temperature and process parameters.
- Each of the inverters 30 shown in FIG. 1, typically comprises an inverter transistor pair as illustrated in FIG. 2.
- a p-channel transistor 32 is coupled in series to the supply voltage Vcc.
- An n-channel transistor 34 is coupled to transistor 32 and to ground. The input is applied to the gates of both transistors 32, 34. The output is taken from the combined drain terminals of the two transistors.
- a low input causes transistor 34 to turn off and transistor 32 to turn on, thus pulling the output high.
- a high input causes transistor 32 to turn off and transistor 34 to turn on, thus pulling the output low.
- a low is defined as a voltage at or close to ground potential and represented by a ⁇ 0 ⁇ .
- a high is defined as a voltage at or close to the supply voltage and is represented by a ⁇ 1 ⁇ .
- oscillation frequency may change by an intolerably large amount due to temperature variations from--55° to 125° C., from supply voltage variations from, for example, 4.5 to 5.5 volts and from variations in semiconductor wafer processing.
- U.S. Pat. No. 5,416,446, issued to Holler, Jr. et al. teaches a technique for programmably generating a frequency using a ring oscillator.
- An operating voltage is developed using a programmable array of field effect transistors. Digitally selecting a given set of the transistors provides a given operating current for the ring, which establishes the frequency of operation.
- U.S. Pat. No. 5,406,228, issued to Hoang, teaches a ring oscillator, logic counter and comparator unit, bias unit, multiplexor and memory.
- the frequency of the ring oscillator is controlled automatically by a settable bias current applied to it by the bias unit.
- the bias unit is controlled by the logic counter and comparator unit which compares the frequency of the ring oscillator with that of an external reference clock signal.
- the bias current is adjusted in accordance with the measured frequency difference. Once the correct frequency has been reached, the digital settings of the bias current are entered into the memory.
- U.S. Pat. No. 5,410,278, issued to Itoh et al., teaches a ring oscillator that comprises a plurality of inverters, a leakage current generating part and a current controlling part.
- the oscillating frequency of the ring oscillator is directly dependent on the amount of leakage current generated from the leakage current generator.
- U.S. Pat. No. 5,446,418, issued to Hara et al., teaches a ring oscillator that includes a plurality of inverters cascaded together.
- Each inverter includes a pair of transistors having certain channel dimensions so as to have an input capacitance for delaying the signal of a preceding stage inverter for a prescribed time period.
- Another object of the present invention to provide a ring oscillator integrated circuit whose frequency of oscillation can be tuned using a trimming step in the course of manufacture.
- a ring oscillator implemented entirely in CMOS technology as part of an integrated circuit (i.e., chip) device.
- a ring oscillator for generating a periodic signal having a predetermined frequency, the periodic signal stable against variations in supply voltage, temperature and process technology
- the ring oscillator including N inverter stages cascaded together, each inverter stage having an input and an output, the output of the N th inverter stage coupled to the input of the first inverter stage, the output of the N th inverter stage forming an output of the ring oscillator, and each the inverter stage including a p-channel transistor having a gate, source and drain, the gate coupled to the input of the inverter stage, the source coupled to a supply voltage, the drain coupled to the output of the inverter stage, the p-channel transistor turning on when the voltage on the gate of the p-channel transistor falls approximately the p-channel threshold voltage plus a delta (V TP + ⁇ ) below the supply voltage, impedance means between the drain of the p-channel transistor and a ground, capacitance
- the capacitance means includes an n-well depletion n-type metal oxide semiconductor (NMOS) transistor and the impedance means comprises a plurality of n-channel transistors, each having a gate, source and drain, the drain coupled to the drain of the p-channel transistor, the source coupled to the ground, the gate coupled to the separate control signal.
- Each the separate control signal comprises the presence or absence of a constant bandgap reference control voltage.
- the ring oscillator according further includes feedback control circuitry having an input and an output, the input of the feedback control circuitry coupled to the output of the N inverter stages, the output of the feedback control circuitry coupled to the input of the N inverter stages, the feedback control circuitry controlling the start/stop operation of the ring oscillator in accordance with a feedback control signal coupled thereto.
- an output buffer having an input and an output, the input of the output buffer coupled to the output of the N th inverter stage, the output of the output buffer forming a buffered output of the ring oscillator.
- the output buffer includes a first inverter having an input, an output, a first and second terminal, the first terminal coupled to the supply voltage, a pull-down impedance coupled between the second terminal and the ground, a second inverter having an input, an output, a third and fourth terminal, the fourth terminal coupled to the ground, the input of the first inverter and the input of the second inverter forming the input of the output buffer, a pull-up impedance coupled between the supply voltage and the third terminal, a second p-channel transistor having a gate, source and drain, the gate of the second p-channel transistor coupled to the output of the first inverter, the source of the second p-channel transistor coupled to the supply voltage, a second n-channel transistor having a gate, source and drain, the gate of the second n
- first, second and third inverter comprises a fifth p-channel transistor in series with a fifth n-channel transistor and the level shifter comprises an n-channel transistor having a gate, the gate of the level shifter coupled to the supply voltage.
- the ring oscillator also includes decoding circuitry for generating the gate control signals, the decoding circuitry which includes a plurality of inverters each having an input and an output, the input of each inverter coupled to an impedance control signal, and a plurality of switches each having an input and an output, the input of each switch coupled to a bandgap reference voltage, the output of each switch forming one of the gate control signals.
- Each of the switches includes a sixth p-channel transistor having a gate, source and drain, the gate of the sixth p-channel transistor coupled to the output of one of the inverters, and a sixth n-channel transistor having a gate, source and drain, the gate of the sixth n-channel transistor coupled to the impedance control signal, the source of the sixth p-channel transistor and the source of the sixth n-channel transistor coupled to the bandgap reference voltage, the drain of the sixth p-channel transistor and the drain of the sixth n-channel transistor forming one of the gate control signals.
- FIG. 1 is a schematic diagram of an example of a prior art ring oscillator constructed using inverters
- FIG. 2 is a detailed schematic diagram illustrating the prior art structure of each inverter section shown in FIG. 1;
- FIG. 3 is a high level block diagram of an example of a ring oscillator constructed in accordance with a preferred embodiment of the present invention
- FIG. 4 is a detailed block diagram of the example of a ring oscillator illustrated in FIG. 3;
- FIG. 5 is a detailed schematic diagram illustrating the single stage and the two stage inverter portions of the ring oscillator of the present invention
- FIG. 6 is a detailed schematic diagram illustrating the pull-down decoding circuitry portion of the ring oscillator of the present invention.
- FIG. 7 is a detailed schematic diagram illustrating the output buffer circuitry portion of the ring oscillator of the present invention.
- FIG. 8 is a detailed schematic diagram illustrating the feedback control circuitry portion of the ring oscillator of the present invention.
- FIG. 9 illustrates the voltage signals that are present at various points within the two stage inverter portion of the present invention.
- Ring oscillator 10 comprises an N stage inverter section 12 whose output is coupled to an output buffer 16 and feedback control circuitry 14. Each stage comprises an inverter transistor pair in series with a current limiting pull-down impedance.
- the output of feedback control circuitry 14 is fed into the input of N stage inverter 12.
- Pull-down decoding circuitry 18 generates control signals used within N stage inverter 12.
- Pull-down decoding circuitry 18 functions to set the frequency of oscillation, as will be explained in further detail, and accepts a number of frequency control signals as input.
- Feedback control circuitry 14 functions to start and stop the oscillation of ring oscillator 10 and receives a number of feedback control signals as input.
- Output buffer 16 functions to take as input the relative low drive output of N stage inverter 12 and outputs a signal able to drive a high capacitance load.
- N stage inverter 12 can comprise any odd integer number N of individual inverter stages. Illustrated in FIG. 4 is a more detailed high level block diagram showing N stage inverter 12 comprising one single stage inverter section 20 and three two stage inverter sections 22. In the example disclosed herein, the number of stages N is equal to 7, however other odd number N inverter stages would also work. Note that there is likely a lower limit to N in that if N is too small, the resulting oscillator will not oscillate. Oscillation will not occur if the delay per stage is of the same magnitude as the total signal propagation time. In other words, if while the output of a stage is still in flux, the output signal has reached around to the input of that particular stage again, the oscillator will not oscillate.
- Each single and two stage inverter section receives pull-down control signals output from pull-down decoding circuitry 18. Also illustrated are feedback control circuitry 14 coupling the output of the last two stage inverter 22 to the input of the first single stage inverter 20 and output buffer 16.
- Two stage inverter section 22 will now be described in more detail.
- a detailed schematic diagram illustrating the single stage and the two stage inverter portions of ring oscillator 10 is shown in FIG. 5.
- two stage inverter section 22 comprises two single stage inverter sections 20 cascaded together, thus the description of the operation of single stage inverter 20 is implied in the description of the operation of two stage inverter section 22.
- the first single stage inverter portion comprises p-channel transistor 40 coupled to an n-channel transistor 42 in a conventional inverter configuration. The gates of transistors 40, 42 are coupled together and receive the input to this inverter stage.
- transistor 40 has a width/length of 2.4/2.4 and transistor 42 has a width/length of 2.4/0.9 (all transistor gate width/length measurements are specified in micrometers). Notwithstanding the dimensions given in the example above, other combinations of width and length are also suitable as would be known by one skilled in the art.
- the source terminal of transistor 42 is coupled to the drain terminals of pull-down n-channel transistors 44, 46, 48.
- the gates of transistors 44, 46, 48 are coupled to signals GATE1, GATE2, GATE3, respectively, and the source terminals are connected to ground.
- the width/length dimensions of transistors 44, 46, 48 are 1/22.5, 1/45, 1/67.5, respectively.
- the drain terminals of transistors 40, 42 comprise the output of the single inverter stage.
- NMOS metal oxide semiconductor
- the second cascaded half of two stage inverter 22 is similar to the first single inverter stage and comprises p-channel transistor 50 and n-channel transistor 52 configured as an inverter pair, pull-down n-channel transistors 54, 56, 58 and n-well depletion NMOS transistor 59.
- the output of inverter transistor pair 40, 42 is coupled to the input of inverter transistor pair 50, 52.
- An n-channel transistor 60 coupled to the input of the second inverter pair 50, 52, functions as a portion of the start/stop circuitry for turning ring oscillator 10 on and off.
- Transistors in the second inverter stage have width/length dimensions identical to corresponding transistors in the first inverter stage.
- two stage inverter section 22 will now be described with reference to FIG. 5 and to FIG. 9 which illustrates the signals present at the input, node A and the output of two stage inverter 22.
- a ring oscillator whose frequency is relatively independent of variations in Vcc, temperature and process technology.
- the ring oscillator circuitry consume relatively little power.
- Each pull-down transistor can be switched into the circuit and when switched, acts as an impedance.
- the drive ratio between p-channel transistor 40 and the drive of each n-channel transistor 44, 46, 48 is relatively high.
- N-channel transistor 42 is basically a short when on, relative to any one of current limiting transistors 44, 46, 48.
- Capacitor 59 i.e. n-well depletion NMOS transistor 59
- Capacitor 49 has been discharged through transistor 42 and one or more of pull-down transistors 44, 46, 48. The asymmetry between transistor 40 and current limiting transistors 44, 46, 48 is reflected in the difference in drive capability between them.
- Transistor 40 has much more drive capacity than any of current limiting transistors 44, 46, 48 due to the difference in transistor geometry. As the input to the inverter transistor pair 40, 42 makes the transition from a ⁇ 1 ⁇ to a ⁇ 0 ⁇ transistor 40 will begin to conduct and transistor 42 will begin to turn off. Transistor 40 will turn on when its gate input voltage drops below the supply voltage by an amount equal to the p-channel threshold voltage V TP . This is in contrast to a conventional inverter transistor pair wherein the effective input switching threshold is typically one half the supply voltage.
- capacitor 49 is quickly charged, thus quickly raising the gate voltage of transistors 50, 52 (i.e. the input to the second inverter stage) to a high level.
- transistor 50 turns off and transistor 52 begins to drive the output low, sinking current from capacitor 59 through the impedance comprising one or more pull-down transistors 54, 56, 58.
- transistor 52 is a relative short.
- the high current limiting impedance of transistors 54, 56, 58 cause capacitor 59 to discharge at a relatively slow rate, much slower than the rate of charging through transistor 50.
- capacitor 59 discharges, the output voltage begins to drop. At the point the output voltage drops to a level V TP below the supply voltage, the p-channel transistor of the next inverter section begins to turn on.
- transistor 42 begins to turn on when the input rises above the n-channel threshold V TN .
- the voltage at node or point A was at a high and begins to fall after the input voltage rises past V TN .
- transitions from ⁇ 0 ⁇ to ⁇ 1 ⁇ occur relatively quickly due to the high drive capability of transistors 40, 50.
- the input to transistor 40 quickly rises past V TP , shutting it off.
- the voltage at node A slowly drops due to the slow discharge of capacitor 49 through transistor 42, which is practically a short by now, and current limiting transistors 44, 46, 48.
- the RC time constant ⁇ is given by: ##EQU1## Where I is the current flowing through transistor 42 and C is the capacitance of transistor 49.
- the key to the present invention and critical to achieving frequency stability of ring oscillator 10, is that transistor 50, of the following inverter section, will not turn on until the voltage on node A drops an amount equal to V TP from the supply voltage, whatever it is, plus a small delta amount.
- the time when the following inverter section is switched is determined not by the supply voltage but by the p-channel threshold V TP and more importantly by the discharge rate time constant RC made up of current limiting transistors 44, 46, 48 and capacitor 49, all of which can be closely controlled during manufacture.
- the time of turn on of the p-channel transistor of the following section is made independent of the supply voltage, since V TP does not vary with the supply voltage.
- the n-channel pull-down transistors function to limit the current through the pair of transistors comprising the inverter.
- the crowbar current in each inverter section is much lower than in conventional inverter circuits but is limited to the current limiting capability of transistor 44, 46, 48. Even at low frequencies wherein both inverter transistors are on for a relatively long period of time, the crowbar current is kept low due to the current limiting n-channel pull-down transistors.
- n-channel transistor 42 does not play a large role in the frequency stability of ring oscillator 10 since it is the discharge rate of capacitor 49 through current limiting transistors 44, 46, 48 that is crucial to the circuit timing.
- Transistor 42 simply acts as a short during when it is on. It does, however, function to reduce the total crowbar current that flows through the inverter during operation of the oscillator. Referring to inverter pair 40, 42, crowbar current does not begin to flow until the input voltage drops low enough to turn on p-channel transistor 40, source current to the input of transistor 50, 52 and cause current to flow through current limiting transistors 44, 46, 48. Transistor 42 has not shut off yet because the input voltage has not dropped below V TN .
- transistor 42 was not in the circuit, crowbar current would flow from the moment transistor 40 turns on until the input transition from a ⁇ 0 ⁇ to a ⁇ 1 ⁇ , shutting off transistor 40, which is a significant amount of time later. With transistor 42 in the circuit, however, crowbar current ceases to flow the moment the input drops below V TN plus a delta which causes transistor 42 to completely shut off.
- the n-channel pull-down or current limiting transistors in each inverter section comprise three n-channel transistors of varying impedance. Other numbers of transistors other than three would also work as well.
- Transistors 44, 46, 48 have a width/length of 1/22.5, 1/45, 1/67.5, respectively.
- the gates of long channel transistors 44, 46, 48 are controlled by three gate signals GATE1, GATE2, GATE3, respectively. Different impedance values can be achieved for the total current limiting impedance by varying the three gate signals. Each gate signal turns one of the three transistors on or off. Using three pull-down transistors provides for eight different impedance settings. Using more than three pull-down transistors gives finer resolution and control over the overall current limiting impedance.
- the gates of the pull-down transistors are preferably driven by a bandgap reference control voltage rather than the regular supply voltage, thus the GATE signals are not dependent on Vcc.
- the bandgap reference is a stable reference voltage independent of temperature, determined by the process technology and typically has a voltage level of approximately 1.3 volts. Bandgap reference circuits are well known in the art and will not be described further.
- Using multiple current limiting transistors having varying gate lengths provides the mechanism by which a stable oscillator frequency across process corners can be achieved. Variations in process technology are removed by programming the pull-down transistors during a calibration or trimming process during manufacture. Different frequencies of oscillation are achieved by varying which pull-down transistors are on (i.e. their impedance added to the total current limiter impedance) and which remain off. Since the frequency of oscillation is determined by the RC time constant ⁇ , if the resistance R is varied, the oscillation frequency will vary accordingly.
- the resistance R can be varied by selectively turning on and off the pull-down transistors. In the example of FIG. 5, three pull-down transistors are shown, thus providing for one of eight resistance values, each resistance value corresponding to a different frequency.
- a trimming process during manufacturing can be used to fine tune the oscillation frequency of the ring oscillator.
- the circuit would initially be produced having the pull-down transistors set in the middle position (e.g., resistance level 4) so as to give the greatest leeway in adjusting the frequency higher or lower.
- the frequency of the ring oscillator is directly measured at an output pad or pin.
- a test mode is designed into the device that is active to switch the oscillator output to a pad or output pin. Based on the difference with the desired frequency, one or more pull-down transistors are put into or taken out of the circuit.
- a trimming process during manufacture of the IC can be used to fine tune the oscillator against process technology variations.
- the frequency of oscillation of conventional ring oscillators is affected by Vcc, temperature and process variation.
- the frequency is also affected by the capacitance of the circuit, the threshold voltage V T of the p-channel transistors, the bandgap reference voltage and the long n-channel current limiting transistors.
- the long n-channel pull-down transistors provide a programmable option and are trimmed during the manufacturing process to fine tune the oscillation frequency.
- the capacitance of the transistors making up a conventional oscillator circuit also affects the oscillation frequency. Variations in the capacitance from transistor to transistor prevents the oscillation frequency from being independent of circuit capacitance. To eliminate the effect of circuit capacitance variations, a capacitance relatively much larger than the capacitance's of the individual circuit components is placed in the circuit loop. The frequency of oscillation becomes dependent instead on the large capacitance, as described previously, rather than the individual transistor capacitance's. Transistors 49, 59 function as large capacitors and are constructed to dwarf the capacitance's of the rest of the circuit. Thus, the frequency of oscillation is now controlled by the geometry and the process of transistors 49, 59, both of which can be managed by suitable design and manufacture, rather than the various capacitance's of the other circuit elements.
- each single inverter section can be constructed in the following manner.
- a p-channel transistor whose gate is coupled to the input to the inverter section, whose source is coupled to the supply voltage and whose drain is the output to the following inverter section.
- Also coupled to the drain are a resistor and capacitor connected in parallel and having a predetermined time constant ⁇ .
- the rate of discharge can also be controlled and it is the rate of discharge of the capacitor that determines when the transistor of the next inverter section turns on, which, in turn, determines the frequency of oscillation.
- the resistor comprises transistors 44, 46, 48 (FIG. 5) and the capacitor comprises transistor 49. Note that it is not necessary that for transistor 42 to be in the circuit for the ring oscillator to work.
- the variations of oscillation frequency due to Vcc are eliminated by using an asymmetrical inverter configuration (i.e., a high drive p-channel transistor in series with a low drive current limiter), as previously described above.
- the p-channel pull-up transistor in each inverter section has a much higher drive capability than the n-channel current limiting transistors. This causes the inverter trip point to be a voltage approximately equal to Vcc-(V TP + ⁇ ), the delta being a value much smaller than V TP itself.
- the threshold voltages V TP and V TN typically vary approximately +/-0.1 V and are generally a function of gate oxide thickness T OX and temperature.
- the programmable n-channel pull-down current limiting transistors remove any variation in frequency due to process variations.
- the delay time of the inverter section is related to the current flow which is given by:
- ⁇ is a non temperature dependent constant of the process technology and both the mobility, ⁇ , and V T are temperature dependent.
- the voltage V GS can be replaced by the bandgap reference voltage, V BGREF , since the current limiting transistors are driven by this gate drive signal.
- the RC time constant ⁇ can then be expressed as: ##EQU2##
- ⁇ can be expressed as: ##EQU3##
- the current term I decreases causing ⁇ to increase.
- the threshold voltage V TN decreases with increasing temperature causing the current I to increase and consequently ⁇ decreases.
- the threshold voltage V TP also decreases with increasing temperature causing ⁇ to decrease.
- the value of the bandgap reference voltage V BGREF can be adjusted so as to achieve an overall canceling effect between the three temperature sensitive parameters.
- Low power consumption is achieved by a combination of techniques, namely: the use of a bandgap reference voltage to reduce transistor gate drive, the use of long channel n-type transistors for the current limiters and the elimination of the crowbar current in output buffer 16, which is described in more detail below.
- An integrated circuit ring oscillator constructed in accordance with the present invention utilizing the above techniques exhibits a voltage independence from 2.7 V to 5.5 V.
- a swing in temperature from -55° to 150° C. results in a variation in frequency of only +/-3 percent.
- Pull-down decoding circuitry 18 will now be described in more detail.
- a detailed schematic diagram illustrating the pull-down decoding circuitry portion of the ring oscillator is shown in FIG. 6.
- the function of pull-down control circuitry is to control the amount of impedance in each current limiter coupled to each inverter section (FIG. 5, i.e. transistors 44, 46, 48).
- Each transistor making up the current limiter has a separate circuit to control its gate signal.
- three gate signals are generated, GATE1, GATE2, GATE3, one for each current limiting transistor.
- the signal applied to the gate of each transistor that is to be turned on is the stable bandgap reference control voltage signal indicated by BANDGAP in FIG. 6.
- Three impedance control signals OSC1, OSC2, OSC3 are coupled to inverters 112, 120, 128, respectively and n-channel pass transistors 116, 124, 132, respectively.
- the output of inverters 112, 120, 128 generate signals OSC1B, OSC2B, OSC3B, respectively and are coupled to the gates of p-channel pass transistors 114, 122, 130, respectively and to the gates of n-channel transistors 118, 126, 134, respectively.
- transistors to be added to the total current limiting impedance For a transistor to be added to the total current limiting impedance, its corresponding input control signal must be high, turning on the n-channel pass transistor and causing the output of the inverter to go low, thus turning on the p-channel transistor and turning off the n-channel pull-down transistor coupled to ground.
- the drain terminals of transistor pairs 114, 116; 122, 124; 130, 132 are coupled together to generate gate control signals GATE1; GATE2; GATE3, respectively.
- the BANDGAP reference signal appears at the corresponding GATE signal output, adding the associated impedance to the total current limiting impedance.
- one or more of the signals OSC1, OSC2, OSC3, is set high. If all are low, feedback control circuitry 14 (FIG. 8) prevents ring oscillator 10 from oscillating.
- Output buffer 16 will now be described in more detail.
- conventional output buffers typically have large crowbar currents associated with them.
- a main object of the present invention is to generate an oscillator signal using as little current as possible.
- the present invention replaces a conventional output buffer circuit with a break before make stage coupled to an output buffer, the combination eliminating any large crowbar currents, drawing relatively little power and providing the high drive needed at the output.
- FIG. 7 A detailed schematic diagram illustrating the output buffer portion of ring oscillator 10 is shown in FIG. 7.
- the function of output buffer 16 is to transform the relatively weak output signal from the final inverter stage into a signal that can drive high capacitive loads.
- the output buffer circuitry must consume as little current as possible.
- two asymmetrical low power inverter stages are used, wherein the traditionally present crowbar current is eliminated by utilizing a break before make technique.
- a slow pull-down 72 and a fast pull-up 70 are utilized in the second inverter stage.
- a fast pull-down 80 and a slow pull-up 78 are utilized.
- the concept is to use a conventional p-channel/n-channel inverter pair but provide separate controls for each transistor.
- Each of the controls comprises an asymmetrical inverter pair in series with a current limiter.
- the p-channel transistor is basically controlled by a fast pull-up transistor and a slow pull-down current limiter.
- the n-channel transistor is basically controlled by a slow pull-up current limiter and a fast pull-down transistor.
- the input to output buffer 16 is coupled to an inverter transistor pair 70, 72 which is in series with a long channel n-type pull-down current limiting transistor 74 whose width/length is 1/40.
- the gate of transistor 74 is coupled to the BANDGAP reference voltage signal.
- the source terminal of transistor 70 is coupled to the supply voltage.
- the output of inverter transistor pair 70, 72 is coupled to the gate of p-channel transistor 82 having gate dimensions of 5/1.1 for high drive purposes.
- the input to output buffer 16 is also coupled to the input of inverter transistor pair 78, 80.
- In series with inverter transistor pair 78, 80 is a long channel p-type pull-up current limiting transistor 76, whose gate is tied to ground and whose dimensions are 1/40.
- the output of inverter transistor pair 78, 80 is coupled to n-channel transistor 84.
- inverter transistor pair 82, 84 is coupled to an n-channel transistor 86 which functions as a level shifter.
- the drain terminal of transistor 86 is coupled to the input of inverter transistor pair 92, 94.
- the dimensions of p-channel transistor 92 and n-channel transistor 94 are 10/1.1 and 5/0.9, respectively.
- the widths of the gates on transistors 92, 94 are large in order to provide high drive currents. Coupled in series between Vcc and the drain terminal of transistor 86 are two p-channel transistors 90, 88.
- output buffer 16 for an input transition from a ⁇ 1 ⁇ to a ⁇ 0 ⁇ to a ⁇ 1 ⁇ will now be described. If the input is initially a ⁇ 1 ⁇ , transistors 70, 78, 84, 92 are off and transistors 72, 80, 82, 94 are on. The input to the level shifter is high and the output of the buffer is a ⁇ 0 ⁇ . The output of the buffer is fed back to the gate of transistor 90. Both transistors 88 and 90 are on, raising the level of the signal from the SUPPLY level to the level of Vcc. Level shifter 86 is necessary in the case where the SUPPLY voltage is less than the Vcc voltage.
- transistor 70 turns on at the point where the input voltage falls to a level V TP + ⁇ below the supply voltage. Although transistor 72 is still on, the crowbar current is minimal due to current limiting transistor 74. Transistor 70, practically a short, drives the gate of transistor 82 high. Although transistor 72 can also be considered a short, transistor 70 provides sufficient current to keep the gate of transistor 82 high, immediately turning it off. Thus, at the voltage V SUPPLY -(V TP + ⁇ ) both transistors 82 and 84 are off. Transistor 78 also turns on but cannot drive transistor 84 because it is current limited by current limiting transistor 76.
- any current flowing through transistor 78 is sunk by transistor 80 which is practically still a short. Only after the input drops to V TN - ⁇ can transistor 80 completely turn off thus allowing transistor 84 to turn on through transistors 76, 78, thus guaranteeing a break before make of transistors 82, 84 respectively.
- Transistor 84 drives the input to level shifter 86 low (i.e., the input to inverter transistor pair 92, 94) causing the output OUT of the output buffer to go high.
- transistor 72 will begin to turn on when the input rises past V TN + ⁇ . Although the impedance of transistor 72 quickly drops, it cannot drive the input to transistor 82 low because transistor 72 is drive limited by current limiter 74. As the input voltage rises, transistor 70 continues to drive the input to transistor 82 high, keeping it off. Transistor 80, however, also begins to turn on as the input voltage rises past V TN + ⁇ but in this case, the drive of transistor 80 is not limited and immediately drives the gate of transistor 84 low, turning it off. Thus, transistor 84 turns off while transistor 82 still remains off. As the input increases, transistor 70 eventually turns off when the input rises past V TP - ⁇ causing the gate of transistor 82 to go low, turning it on. Thus, a break before make is also guaranteed for a ⁇ 0 ⁇ to ⁇ 1 ⁇ transition.
- Transistor 82 turning on drives the input to level shifter 86 high.
- Level shifter 86 is needed to prevent the p-type drain and the n-type body (or substrate) of transistor 82 from becoming a forward biased diode and destroying the operation of the output buffer. This is likely to happen if the supply voltage is lower than Vcc. Due to the V GS drop across the level shifter, the voltage level input to inverter pair 92, 94 would be limited to V SUPPLY -V TN , limiting the drive of the final output stage leaving the inverter pair 92, 94 partially on and defeating the goal of low current. Transistor 90 acts to raise the voltage of the input to inverter pair 92, 94 to Vcc when the output OUT is low, thus causing transistor 94 to be driven fully.
- transistor 88 The function of transistor 88 will now be described. As previously described, as the input to the output buffer changes from a ⁇ 1 ⁇ to a ⁇ 0 ⁇ , the output of transistor 70 goes from a low to a high, turning off transistor 82. In addition, transistor 78 goes from a low to a high turning on transistor 84. At this point in time, however, the output OUT is still low, keeping transistor 90 on. Without transistor 88, crowbar current flows through the path including transistors 90, 86 and 84. Transistor 90 cannot shut off immediately because of the inherent delay in the signal through the final output inverter stage. Crowbar current continues to flow up until transistor 94 turns off and transistor 92 turns on, at which point transistor 90 turns off.
- transistor 88 functions to eliminate the crowbar current by eliminating the current flow through transistor 90.
- Transistor 88 turns off at the same time transistor 82 shuts off, guaranteeing that no crowbar current can flow through transistor 84. This is because, as explained above, transistor 82 shuts off before transistor 84 turns on due to their different gate drive signal.
- Feedback control circuitry 14 will now be described in more detail.
- a detailed schematic diagram illustrating the feedback control circuitry portion of ring oscillator 10 is shown in FIG. 8.
- Feedback control circuitry 14 comprises n-channel transistor 106, p-channel transistor 108, n-channel transistor 100 coupled to ground, NAND gates 102, 104 and inverter 110.
- the output signal from the last two stage inverter 22 (FIG. 4) is fed back to the source terminals of transistors 106, 108.
- the function of feedback control circuitry 14 is to provide on/off oscillation control of ring oscillator 10.
- a RUN signal provides the start and stop signal for the oscillator. When RUN is low the oscillator cannot operate, when high the oscillator is allowed to operate.
- the signals OSC1B, OSC2B, OSC3B are generated by pull-down decoding circuitry 18, described in more detail below.
- RUN For ring oscillator 10 to be on, RUN must be high and at least one of the inputs to NAND gate 102 must be low.
- the inputs to NAND gate 104 are thus high and RUNB is low, turning on transistor 108 and turning off transistor 100.
- the output of inverter 110 is high, turning on transistor 106. With transistors 106, 108 both on, the feedback signal from the last two stage inverter section 22 is passed through to the input of single stage inverter 20 (FIG. 4).
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Abstract
Description
I=βμ(T) V.sub.GS -V.sub.T (T)!.sup.2
Claims (12)
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US08/639,281 US5796313A (en) | 1996-04-25 | 1996-04-25 | Low power programmable ring oscillator |
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US08/639,281 US5796313A (en) | 1996-04-25 | 1996-04-25 | Low power programmable ring oscillator |
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